Head-mounted display eyeball tracker integrated system

ABSTRACT

Head-mounted display with an eye-tracking system and including a light-transmitting substrate ( 20 ) having two major surfaces and edges, optical means for coupling light into said substrate ( 20 ) by total internal reflection, partially-reflecting surfaces ( 22   a - 22   c ) carried by the substrate ( 20 ) that are not parallel with the major surfaces of the substrate ( 20 ), a near-infrared light source ( 78 ) and a display source ( 92 ) projecting within the photopic spectrum, wherein light from the light source ( 78 ) and light from the display source ( 92 ) are coupled into the substrate ( 20 ) by total internal reflection.

FIELD OF THE INVENTION

The present invention relates to integrated head-mounted display (HMD)systems, and in particular, to systems that include two combined units:a head-mounted unit and an eyeball tracking unit.

The invention can be implemented to advantage in a large number ofimaging applications, such as portable DVDs, cellular phones, mobile TVreceivers, video games, portable media players or other mobile displaydevices.

BACKGROUND OF THE INVENTION

One important application for compact optical elements, is in HMDswherein an optical module serves both as an imaging lens and a combiner,in which a two-dimensional image source is imaged to infinity andreflected into an eye of an observer. The display source can be obtaineddirectly from, e.g., a spatial light modulator (SLM) such as a cathoderay tube (CRT), a liquid crystal display (LCD), an organic lightemitting diode array (OLED), a scanning source, or indirectly, by meansof a relay lens, or an optical fiber bundle. The display sourcecomprises an array of elements (pixels) imaged to infinity by acollimating lens and transmitted into the eye of a viewer by means of areflecting or partially reflecting surface acting as a combiner fornon-see-through and see-through applications, respectively. Typically, aconventional, free-space optical module is used for these purposes. Asthe desired field-of-view (FOV) of a HMD system increases, however, sucha conventional optical module becomes larger, heavier and bulkier, andtherefore, even for a moderate-performance device, is impractical. Thisis a major drawback for all kinds of displays and especially inhead-mounted applications, wherein the system should necessarily be aslight and compact as possible.

The strive for compactness has led to several different complex opticalsolutions, all of which, on the one hand, are still not sufficientlycompact for most practical applications and, on the other hand, suffermajor drawbacks in terms of manufacturability. Furthermore, theeye-motion-box (EMB) of the optical viewing angles resulting from thesedesigns is usually very small, typically less than 8 mm. Hence, theperformance of the optical system is very sensitive, even for smallmovements of the optical system relative to the eye of a viewer, anddoes not allow sufficient pupil motion for comfortable reading of textfrom such displays.

The teachings included in Publication Nos. WO01/95027, WO03/081320,WO2005/024485, WO2005/024491, WO2005/024969, WO2005/124427,WO2006/013565, WO2006/085309, WO2006/085310, WO2006/087709,WO2007/054928, WO2007/093983, WO2008/023367, WO2008/129539 andWO2008/149339, all in the name of Applicant, are herein incorporated byreferences.

DISCLOSURE OF THE INVENTION

The present invention facilitates the exploitation of very compactlight-guide optical elements (LOEs) for, amongst other applications,HMDs. The invention allows for relatively wide FOVs together withrelatively large EMB values. The resulting optical system offers alarge, high-quality image, which also accommodates large movements ofthe eye. The optical system offered by the present invention isparticularly advantageous because it is substantially more compact thanthe state-of-the-art implementations and yet, it can be readilyincorporated even into optical systems having specializedconfigurations.

Another optical function which could prove to be useful for HMD designsis eyeball tracking, or sensing the direction the eyeball is looking at,relative to the direction of the head. A typical eye tracker willcombine a miniature CCD camera and an infrared LED to illuminate thepupil. By measuring the changes in shape and position of the pupil, itis possible to perceive the direction in which the viewer's eye islooking, with very reasonable accuracy once calibrated. Combiningmeasurements of head position and eye position would solve the problemsinherent in existing HMD technology, since the projected symbols andboresight could be slaved to the direction in which the viewer islooking, thus retaining existing human tracking behavior. It will beuseful to combine the HMD and the eyeball tracker in the same opticalmodule.

A broad object of the present invention is therefore to alleviate thedrawbacks of prior art compact optical display devices and to provideother optical components and systems having improved performance,according to specific requirements.

In accordance with the invention there is therefore provided an opticalsystem, comprising a light-transmitting substrate having at least twomajor surfaces and edges, at least one optical means for coupling lightwaves into the substrate by total internal reflection, at least twopartially reflecting surfaces carried by the substrate wherein thepartially reflecting surfaces are not parallel to the main surfaces ofthe substrate, at least one light source projecting light waves locatedwithin a first optical spectrum, and at least one display sourceprojecting light waves located within a second optical spectrum,characterized in that the light waves from the light source and lightwaves from the display source are coupled into the substrate by totalinternal reflection.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in connection with certain preferredembodiments, with reference to the following illustrative figures sothat it may be more fully understood.

With specific reference to the figures in detail, it is stressed thatthe particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention. The description taken with the drawings are to serve asdirection to those skilled in the art as to how the several forms of theinvention may be embodied in practice.

In the drawings:

FIG. 1 is a side view of an exemplary, prior art, LOE;

FIGS. 2A and 2B illustrate desired reflectance and transmittancecharacteristics of selectively reflecting surfaces, used in the presentinvention, for two ranges of incident angles;

FIG. 3 illustrates a reflectance curve as a function of the incidentangle for an exemplary dielectric coating;

FIG. 4 is a schematic diagram illustrating a detailed sectional view ofan exemplary array of selectively reflective surfaces;

FIG. 5 illustrates a prior art eyeglass HMD device;

FIG. 6 illustrates a side view of an LOE showing light waves scatteredfrom an eye and coupled back into the LOE;

FIG. 7 illustrates a reflectance curve as a function of the wavelengthfor a specific partially reflecting surface;

FIG. 8 illustrates a reflectance curve as a function of the wavelengthfor another partially reflecting surface;

FIG. 9 illustrates reflectance curves as a function of the incidentangle for two different partially reflecting surfaces;

FIG. 10 illustrates a side view of an LOE, showing light waves coupledout through an exit aperture, which are scattered from the eye andcoupled back into the LOE;

FIG. 11 illustrates a side view of an LOE, showing light waves which arescattered from an eye and coupled back into the LOE, wherein only partof the rays are coupled out through an exit aperture;

FIG. 12 illustrates reflectance curves as a function of the wavelengthfor a reflection filter at two different incident angles;

FIG. 13 illustrates reflectance curves as a function of the wavelengthfor a transmission filter at two different incident angles;

FIG. 14 illustrates an optical system combining light waves from adisplay source and a light source;

FIG. 15 illustrates a reflectance curve as a function of the wavelengthfor a polarizing beam splitter, and

FIG. 16 illustrates a side view of still another embodiment of an LOEhaving two adjacent partially reflecting surfaces for coupling out lightwaves into the viewer's eye.

FIG. 17 illustrates a side view of another embodiment of an LOE havingtwo separate coupling in elements.

FIG. 18 illustrates a reflective pattern of the polarizing beam splitterof FIG. 17.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a sectional view of a prior art substrate 20 andassociated components (hereinafter also “an LOE”), utilizable in thepresent invention. An optical means, e.g., a reflecting surface 16, isilluminated by a collimated display 18, emanating from a light source(not shown) located behind the LOE. The reflecting surface 16 reflectsincident light from the source, such that the light is trapped inside aplanar substrate 20 of the LOE, by total internal reflection. Afterseveral reflections off the major lower and upper surfaces 26, 27 of thesubstrate 20, the trapped waves reach an array of selective reflectingsurfaces 22, which couple the light out of the substrate into an eye 24,having a pupil 25, of a viewer. Herein, the input surface of the LOEwill be regarded as the surface through which the input waves enter theLOE and the output surface of the LOE will be regarded as the surfacethrough which the trapped waves exit the LOE. In the case of the LOEillustrated in FIG. 1, both the input and the output surfaces are on thelower surface 26. Other configurations, however, are envisioned in whichthe input and the image waves could be located on opposite sides of thesubstrate 20. Assuming that the central wave of the source is coupledout of the substrate 20 in a direction normal to the substrate surface26, the reflecting surfaces 22 are flat, and the off-axis angle of thecoupled waves inside the substrate 20 is α_(in), then the angle α_(sur2)between the reflecting surfaces and the normal to the substrate planeis:

$\begin{matrix}{\alpha_{{sur}\; 2} = {\frac{\alpha_{in}}{2}.}} & (1)\end{matrix}$

As seen in FIGS. 1 and 2A, the trapped rays arrive at the reflectingsurfaces from two distinct directions 28, 30. In this particularembodiment, the trapped rays arrive at the reflecting surface from oneof these directions 28 after an even number of reflections from thesubstrate surfaces 26 and 27, wherein the incident angle β_(ref) (seeFIG. 2A) between the trapped ray and the normal to the reflectingsurface is:

$\begin{matrix}{\beta_{ref} = {{{90{^\circ}} - ( {\alpha_{in} - \alpha_{{sur}\; 2}} )} = {{90{^\circ}} - {\frac{\alpha_{in}}{2}.}}}} & (2)\end{matrix}$

The trapped rays arrive at the reflecting surface from the seconddirection 30 after an odd number of reflections from the substratesurfaces 26 and 27, where the off-axis angle is a α′_(in)=180°−α_(in)and the incident angle between the trapped ray and the normal to thereflecting surface is as indicated in FIG. 2B:

$\begin{matrix}{\beta_{ref}^{\prime} = {{{90{^\circ}} - ( {\alpha_{in}^{\prime} - \alpha_{{sur}\; 2}} )} = {{{90{^\circ}} - ( {{180{^\circ}} - \alpha_{in} - \alpha_{{sur}\; 2}} )} = {{{- 90}{^\circ}} + {\frac{3\alpha_{in}}{2}.}}}}} & (3)\end{matrix}$

As further illustrated in FIG. 1, for each reflecting surface, each rayfirst arrives at the surface from the direction 30, wherein some of therays again impinge on the surface from direction 28. In order to preventundesired reflections and ghost images, it is important that thereflectance be negligible for the rays that impinge on the surfacehaving the second direction 28.

A solution for this requirement that exploits the angular sensitivity ofthin film coatings was previously proposed in the Publicationsreferred-to above. The desired discrimination between the two incidentdirections can be achieved if one angle is significantly smaller thanthe other one. It is possible to provide a coating with very lowreflectance at high incident angles, and a high reflectance for lowincident angles. This property can be exploited to prevent undesiredreflections and ghost images by eliminating the reflectance in one ofthe two directions. For example choosing β_(ref)˜25°, then it can becalculated that:

β′_(ref)=105°;α_(in)=50°;α′_(in)=130°;α_(sur2)=25°.  (4)

If a reflecting surface is now determined for which β′_(ref) is notreflected but β_(ref) is, then the desired condition is achieved.

Referring now specifically to FIGS. 2A and 2B, these figures illustratedesired reflectance behavior of selectively reflecting surfaces. Whilethe ray 32 (FIG. 2A), having an off-axis angle of β_(ref)˜25°, ispartially reflected and is coupled out of the substrate 34, the ray 36(FIG. 2B), which arrives at an off-axis angle of β_(ref)′˜75° to thereflecting surface (which is equivalent to β_(ref)′˜105°, is transmittedthrough the reflecting surface 34, without any notable reflection. AnLOE is usually exploited not only for a single wave, but for an opticalsystem having a wide FOV. Assuming a system having a FOV of 30° and anLOE having a refractive index of 1.517, then the FOV inside thesubstrate is ˜20°. As a result, there are two angular regions which aredefined for this specific LOE: a first region of 75°±10° where β′_(ref)is located, and a second region of 25°±10° where β_(ref) is located.

FIG. 3 illustrates the reflectance curve of a typical partiallyreflecting surface of this specific LOE, as a function of the incidentangle for S-polarized light with the wavelength λ=550 nm. For afull-color display, similar reflectance curves should be achieved forall the other wavelengths in the photopic region. There are twosignificant regions in this graph: between 65° and 85°, where thereflectance is very low, and between 10° and 40°, where the reflectanceincreases monotonically with increasing incident angles. Hence, as longas, for a given FOV and for a given spectral region, it can ensured thatthe entire angular spectrum of β′_(ref), where very low reflections aredesired, will be located inside the first region, while the entireangular spectrum of β_(ref), where higher reflections are required, willbe located inside the second region, the reflection into the viewer'seye of an embodiment having only one substrate can be ensured, thusensuring a ghost-free image.

FIG. 4 is a schematic sectional view of an array of selectivelyreflecting surfaces which couple light rays trapped inside the substrateout and into an eye of a viewer. As can be seen, in each cycle, thecoupled rays pass through reflecting surfaces 38, having a direction ofα′_(in)=130°, whereby the angle between the rays and the normal to thereflecting surfaces, is ˜75°, and the reflections from these surfacesare negligible. In addition, in each cycle, the rays 39 pass through thereflecting surface 22 twice in a direction of α_(in)=50°, where theincident angle is 25° and part of the energy of the ray is coupled outof the substrate.

In general, all the potential configurations of the LOEs considered inthe Publications referred-to above, offer several important advantagesover alternative compact optics for display applications, which includethat:

1) the input display source can be located very close to the substrate,so that the overall optical system is compact and lightweight, offeringan unparalleled form-factor;

2) in contrast to other compact display configurations, the LOEtechnology offers flexibility as to location of the input display sourcerelative to the eyepiece. This flexibility, combined with the ability tolocate the display source close to the expanding substrate, alleviatesthe need to use an off-axis optical configuration that is common toother display systems. In addition, since the input aperture of the LOEis much smaller than the active area of the output aperture, thenumerical aperture of the collimating lens is much smaller than requiredfor a comparable conventional imaging system. Consequently, asignificantly more convenient optical system can be implemented and themany difficulties associated with off-axis optics and highnumerical-aperture lenses, such as field or chromatic aberrations, canbe compensated-for relatively easily and efficiently;

3) the reflectance coefficients of the selectively reflecting surfacesin the present invention, are essentially identical over the entirerelevant spectrum. Hence, both monochromatic and polychromatic lightsources may be used as display sources. The LOE has a negligiblewavelength-dependence, ensuring high-quality color images with highresolutions;

4) since each point from the input image is transformed into a planelight wave that is reflected into the eye of a viewer from a large partof the reflecting array, the tolerances on the exact location of the eyecan be significantly relaxed. As such, the viewer can see the entireFOV, and the EMB can be significantly larger than in other compactdisplay configurations, and

5) since a large part of the intensity from the display source iscoupled into the substrate, and since a large portion of this coupledenergy is “recycled” and coupled out into an eye of a viewer, a displayof comparatively high brightness can be achieved even with displaysources having low-power consumption.

FIG. 5 illustrates a prior art embodiment in which the LOE is embeddedin eyeglass frames 40. The display source 42 and the collimating device44, which includes a light waves folding element, are assembled insidearm portions 46 of the eyeglass frames 40 next to the edge of the LOE.In a case where the display source is an electronic element, such as asmall CRT, LCD or OLED, driving electronics 48 for the display source,may be assembled with the back portion of the arm 46. A handheld unit 50comprising a power supply, a video source and control interface isconnected to arm 46 by a cable 52, which is used for transmitting power,video signals, audio signals and control commands. Earphones can also beinstalled in the eyeglasses to enable the exploitation of audiochannels. The handheld unit 50 can be a portable DVD, a cellular phone,a mobile TV receiver, a video games console, a portable media player, orany other mobile display device. The unit 50 is referred to as“handheld”, since it is usually operated by the user's hand, but it canbe any other portable device, and it can be affixed to the user's beltor located in a pocket, a pouch, a purse or hung on the user's neck. Inaddition to the components which are embedded in the eyeglass frame, aminiature video camera 54 with, optional optical zoom capability, can beinstalled e.g., in the front region of the frame 40. The camera capturesimages from the external scene, transfers the video signal to animage-processing unit 56, which can be installed inside the electronicsunit 48, and controlled in real-time by the user. The processed imagesignal is then transferred to the display source 42 which projects theimage through the LOE into the eye of a viewer. Other potential elementsthat can be installed on the frame are a GPS receiver, an orientationsensor and a coordinate position sensor, wherein the processor 56receiving an input from these sensors is providing a visually sensibleoutput for displaying on the eyeglass.

Some of the current HMD technology uses head position measurements toapproximate line-of-sight, which may cause significant disparity betweenwhat a viewer is intended to look at, and what the viewer is actuallylooking at, as a result of at least ±20° eye movement. Therefore, it isnecessary to integrate eyeball tracking capability into HMDs in someapplications. Eyeball tracking is the process of measuring either thepoint of gaze or the motion of an eye relative to the head. An eyeballtracker is a device for measuring eye positions and eye movement. Themost popular method for operating this device is by utilizing an opticalmethod for measuring eye motion. Light from an external source,typically infrared, is reflected from the eye and sensed by a videocamera, or some other specially designed optical sensors. Theinformation is then analyzed to extract eye rotation from changes inreflections. Video-based eye trackers typically use corneal reflectionand the center of the pupil as features to track over time. As a result,an HMD-eyeball tracker integrated system would be able to displaystereoscopic virtual images as would a classical HMD, and also be ableto track the ‘direction of gaze’ of a viewer.

In accordance with the present invention, it would be advantageous tophysically combine the two optical units, the HMD and the eyeballtracker. Moreover, it would be beneficial to utilize the same LOE forprojecting the light from the display source into a viewer's eye, asdescribed above, as well as for illuminating the eye with light from theeye tracker source, and to collect light which reflects from the eyeinto the detector. These two optical units should work properly withoutinterfering with each other. To achieve this goal, two maincharacteristics of the combined optical system are exploited in thepresent invention: a separate partially reflecting surface or facet,dedicated for transferring light from a light source to the inspectedeye and backwards, and a light having a wavelength substantiallydifferent from the photopic region utilized for the eye tracking.

FIG. 6 schematically illustrates how one of the surfaces of an LOE canbe utilized to illuminate the viewer's eye 24 for eye-tracking purposes.As illustrated, light rays from an eyeball tracker 64 having awavelength of λtr, which is substantially different than the photopicregion, usually in the near IR region and preferably at the range of850-900 nm, are coupled into the LOE by total internal reflectionthrough the light waves coupling surface 16. In this embodiment, theinput and the image waves are located on opposite sides of the LOE. Thelight waves are coupled-out of the LOE by the partially reflectingsurface 22 a and are directed to illuminate the viewer's eye 24. Afterreflecting from the eye 24, rays 60 and 62 are coupled back into the LOEby the same partially reflecting surface 22 a and then coupled-out ofthe LOE by the surface 16, back into the eye tracker 64, wherein lightwaves are imaged by a detector 66, which analyzes the incoming rays totrack the position of the eye-pupil 25.

In order to avoid ghost images, it is important that only one of thefacets of the surfaces of the LOE (partially reflecting surface 22 a inthe shown Figure) will reflect light waves in the range of about λir.Otherwise, light waves from other surfaces will also be reflected fromthe eye and cause a noise on the detector 66, thus severely degradingthe quality of the imaged eyeball. In addition, reflecting surface 22 ashould be transparent to the photopic range in the relevant angularspectra of the LOE, in the lower region, as well as the upper one.

FIGS. 7 and 8 illustrate the reflectance curve of partially reflectingsurfaces 22 a and 22 b, respectively, at incident angles of 35° and 75°as a function of the wavelength. As shown in FIG. 7, reflecting surface22 a reflects light waves having a wavelength of 850 nm with areflectance of 20% at an incident angle of 35°, while it is actuallytransparent for the entire photopic range at both incident angles. At anincident angle of 35°, reflecting surface 22 b partially reflects thelight waves in the photopic range while it is actually transparent at anincident angle of 75° in the photopic range, as well as at both angles,in the region of 850 nm.

FIG. 9 illustrates the reflectance curve of partially reflectingsurfaces 22 a and 22 b at a wavelength of 550 nm as a function of theincident angle. As illustrated, for surface 22 b there are twosignificant regions in this graph: between 65° and 85°, where thereflectance is very low, and between 10° and 40°, where the reflectanceincreases monotonically with increasing incident angles, as required forthe regular operation of an LOE. For partially reflecting surface 22 athe reflectance is negligible at both lower and higher relevant angularregions. The actual interpretation of FIGS. 7 to 9 is that reflectingsurface 22 a is solely dedicated for eyeball tracking and does not atall interfere with the usual operation of the LOE at the photopicregion. In addition, reflecting surface 22 b is substantiallytransparent to the spectral region of around 850 nm, and hence, does notinterfere with the optical operation of eyeball tracking. All the otherpartially reflecting surfaces are designed to behave in a similar mannerto that of reflecting surface 22 b. That is, the other facets are alsotransparent to the spectral region of 850 nm and have opticalperformance in the photopic region, as required by the optical design ofthe LOE acting as a combiner for HMD.

Another problem that should be addressed is the possibility that a ghostimage might also for a single surface. As illustrated in FIG. 10, twodifferent rays from a single point in the eye 24 are imaged through theLOE. Their optical behavior is, however, different: while ray 68 isreflected only once from each of the external surfaces 28 and 30 of theLOE, the ray 70 is reflected twice from each surface. As a result, thesetwo rays have different optical pathways from partially reflectingsurface 22 a to reflecting surface 16, and hence, they cannot beutilized together to form the image of wavelength λout at the detector66. Therefore, the rays that are reflected twice from the externalsurfaces 28, 30 must be blocked from the detector 66.

FIG. 11 illustrates how a spatial filter 72 which is located at thereflecting surface 16, blocks these undesired rays. That is, ray 74 isno longer reflected by surface 16 via tracker 64 into the detector 66,but rather continues to propagate inside the substrate, and iseventually absorbed at the edges of the LOE. In order to avoiddisturbance for the incoming light waves from the display source whichcreates the image that is projected by the LOE into a viewer's eye, itis important that the filter 72 should be transparent to the light waveshaving a wavelength of λtr, while still being reflective to the photopicrange.

FIG. 12 illustrates the reflectance curve of the filter 72 as a functionof the wavelength at incident angles of 15° and 35°. As illustrated, thefilter is highly reflective for the photopic range, while it issubstantially transparent for the spectral range around 850 nm.

The embodiments described above with regard to the reflecting surface 16are examples of a method for coupling the input waves into thesubstrate. Input waves could, however, also be coupled into thesubstrate by other optical means, including, but not limited to, foldingprisms, fiber optic bundles, diffraction gratings, and other solutions.In some of these methods, which were described in the Publicationsreferred to above, the input surface of the LOE is not a reflectingsurface but rather a transparent aperture. In these cases, it isrequired that the filter will be reflective to light waves having awavelength of λtr, while still transparent to the photopic range.

FIG. 13 illustrates the reflectance curve of filter 72 as a function ofthe wavelength at incident angles of 15° and 35°. As illustrated, thefilter 72 is substantially transparent for the photopic range, while itis reflective for the spectral range of about 850 nm.

The combination of a display source with a light source for illuminatingan eye tracker utilizing light waves having a wavelength of λtr, isillustrated in FIG. 14. As shown, the s-polarized input light waves 80emanating from the light source 78 and having wavelengths inside thephotopic spectrum, are reflected by a reflective dichroic surface 82,e.g., a beam splitter, associated with a light guide 83, and are thencoupled into a light guide 84 of a combiner 85, usually composed of alight waves transmitting material, through its lower surface 86.Following reflection of the light waves off of a polarizing beamsplitter 88, the light waves are coupled out of the light guide 84through surface 90. The light waves which illuminate bright pixels ofthe Liquid Crystal on Silicon (LCOS) 92 then pass through a dynamicquarter-wavelength retardation plate 94, reflected by a reflectingsurface of the LCOS 92, return to pass again through the retardationplate 94, and re-enter the light-guide 84 through surface 90. The nowp-polarized light waves pass through the polarizing beam splitter 88 andare coupled out of the light guide 84 through surface 96. The lightwaves then pass through a second quarter-wavelength retardation plate98, collimated by a component 100, e.g., a lens, at its reflectingsurface 102, return to pass again through the retardation plate 98, andre-enter the light-guide 84 through surface 96. The now s-polarizedlight-waves reflect off the polarizing beam splitter 88 and exit thelight guide through the upper surface 104 of the combiner 85. Inaddition, s-polarized input light waves 106 having wavelengths of thelight illuminating source 107, located in the eyeball tracker 108 andhave an optical spectrum different than the photopic spectrum,preferably, in the near IR region, pass through the dichroic surface 82,are coupled into a light guide 84, pass directly through the polarizingbeam splitter 88 and are then coupled out of the light-guide 84 throughthe upper surface 104. For the relevant angular spectrum, the dichroicsurface 82 has high reflectance for the photopic spectrum and hightransmittance for the spectrum of the light waves 106.

The two spectrally separated s-polarized input light waves 80 and 106are now coupled through the reflecting surface 16 of the LOE, by totalinternal reflection. The light waves 80 are utilized for forming avirtual image projected by partially reflecting surfaces 22 a-22 e intoa viewer's eye 24, while the light waves 106 are utilized to illuminatethe eye 24 for eye-tracking. The light waves 106 having the wavelengthof λtr, are reflected from the eye 24 coupled again into the LOE by thepartially reflecting surface 22 a, coupled out from the LOE throughreflecting surface 16, and as seen, pass again through the polarizingbeam splitter 88 and through the dichroic surface 82, and coupled intothe eyeball tracker 108, where they are focused onto the detector 110.

FIG. 15 illustrates the reflectance pattern of the polarizing beamsplitter 88 of FIG. 14. As shown, the beam splitter 88 has high and lowreflectance, for the s- and p-polarization, respectively, in thephotopic range, while having high transmittance for the s-polarizedlight having a wavelength of λtr.

In all the configurations described so far, the optical reflecting 16 isutilized to couple light waves from the display source havingwavelengths in the photopic range, as well as light waves from theeyeball tracker 108 having wavelength of λtr, into the LOE, by totalinternal reflection. There are, however, configurations whereindifferent coupling elements are utilized to couple separately the lightwaves from the display source and the light waves from the eyeballtracker. These configuration include, but are not limited to, twodifferent elements wherein the first one is substantially transparentfor the photopic range, while it is reflective for the spectral rangewavelength of Air, and the second element is substantially transparentfor the spectral range wavelength of λtr, while it is reflective forphotopic range.

In all the configurations described so far, the two partially reflectingsurfaces, 22 a and 22 b, are laterally separated. However, there areconfigurations wherein, for the sake of compactness or for enlarging theEMB of the optical system, it is required that the two surfaces will beadjacent to each other.

FIG. 16 illustrates a first partially reflecting surface 22 a which iscoated on the surface directed to one side of substrate 20, and a secondpartially reflecting surface 22 b which is coated on the surfacedirected towards the other side of substrate 20. The two surfaces areoptically attached and laterally separated by a cement layer 112.Typically, the thickness of the cement layer 112 is in the order of 10μm, hence, the two surfaces can be considered as being opticallydisposed in one location. As illustrated, two different rays 116, 118from the eyeball tracker 108 and the display source 92, respectively,(see FIG. 14) are coupled into the substrate 20 by the couplingreflecting surface 16 and then coupled out of the substrate 20 towardsthe viewer's eye 24 by partially reflecting surfaces 22 a and 22 b,respectively.

In all the hereinbefore described embodiments, the light waves from theeyeball tracker, as well as from the display source, are coupled intothe substrate by the same coupling-in element. However, there areembodiments wherein, for the sake of simplicity or because ofgeometrical constraints, it is required that the eyeball tracker and thedisplay source will be separated, and hence, the two different lightwaves will impinge on the substrate at two different locations.

FIG. 17 illustrates a system wherein the optical waves 116 and 118 fromthe eyeball tracker 120 and the display source 122, are separatelycoupled into the substrate 20 by two different coupling-in elements, 124and 126, respectively. While the coupling-in element 124 is a simplereflecting surface, the coupling-in element 126 is a dichroic beamsplitter. It is assumed that the angle between the surfaces 124 and 126and the major surface 26 of the substrate 20 is about 30°.

FIG. 18 illustrates a reflection pattern of the polarizing beam splitter126 of FIG. 17. As shown, the beam splitter 126 has high reflectance forthe s-polarization in the photopic range at an incident angle of 30°,while having high transmittance for the s-polarized light having awavelength of λtr at the same angle. As a result, light waves 116,having a wavelength of λtr, are coupled into the substrate by thereflecting surface 124 and then pass through the element 126 withnegligible interference.

In some of methods described in the prior art Publications referred toabove, the input surface of the LOE is not a reflecting surface, butrather a transparent aperture. In these cases, it is required that thesecond aperture will be reflective to light waves having a wavelength ofλtr while still being transparent to the photopic range.

So far, it was assumed that the main purpose of the eyeball tracker isto measure eye positions and eye movements. When, however, an eyelid ofa viewer's eye is closed, the pattern of the optical waves which arereflected from the eye, is significantly changed. The eyeball trackercan easily detect if a viewer's eyelid is open or closed. Since theLOE-based eyeglasses illustrated in FIG. 5 have a see-throughcapability, it is possible to utilize same for automotive applicationswhere they can potentially assist a drive in driving and in navigationtasks, or can project a thermal image in the driver's eyes duringlow-visibility conditions. In addition to these tasks, the LOE-basedeyeglasses, combined with an eyeball tracker, can also serve as a drowsydriver alert unit, that is, the eyeball tracker device can detect thedriver's blinking patterns and determine how long the driver's eyes stayclosed between blinks. Hence, the system can conclude that the driver isno longer alert and provide a warning concerning this situation.

What is claimed is:
 1. A see-through optical system for vehicle safety,comprising: a transparent substrate having at least two major surfacesand edges; an image projector configured to project an image made up oflight waves of at least a second optical spectrum, said image projectorbeing optically coupled to the substrate so as to inject the image intothe substrate so that the image propagates within the substrate byinternal reflection at the two major surfaces; an array ofpartially-reflective surfaces carried by said substrate and obliquelyangled relative to said major surfaces, said array ofpartially-reflective surfaces coupling out the image from the substrateso as to be viewable by the eye of a viewer; an illumination sourcegenerating illumination light waves within a first optical spectrumdistinct from said second optical spectrum; a chromatically-selectivereflector carried by the substrate and configured for coupling-in lightwaves generated by said illumination source reflected from the eye ofthe viewer; a detector arrangement deployed to derive an image of atleast part of the eye of the viewer from said light waves reflected fromthe eye of the viewer; and a processing system associated with saiddetector arrangement and configured to process said image of at leastpart of the eye of the viewer to detect an open/closed state of the eyeand to derive an indication of driver drowsiness.